We studied human T cell activation and regulation in the system of continued cultures of T cells that respond to T cell growth factor (TCGF). T cells cooperate with monocytes and with B cells during lymphoproliferation to the lectin phytohemagglutinin. Monocytes and a few myeloid and monocytic leukemia cell lines function as assessory cells and produce interleukin 1 (IL-1). We studied the TCGF producing cells, and cells responding to TCGF, by means of anti-T cell sera, Percoll gradients, and the presence of receptors for the Fc portion of IgG (FcgammaR). Cultured T cells (CTC) are cytotoxic in antibody-dependent cell-mediated cytotoxicity, natural cytotoxicity and specific immune cytolysis. Lymphocytes from cancer patients were sensitized to their autologous tumor cells, expanded as CTC and found to react specifically in cytotoxicity and by proliferation to the original stimulating tumor cells.